The union wants MPs to block Brexit unless the Government can meet an extensive list of guarantees about workers rights.

The participation of a union which supports Mr Corbyn in the case appears to contradict claims by the Labour leadership that the party will not seek to block Article 50. John McDonnell, the shadow chancellor, said earlier this week: "Labour will not seek to block or delay it."

Supreme Court justices will also invite legal representatives from the Welsh government and a group representing British expats to intervene in the court case, which will begin on December 5.

Downing Street says it is confident it will win its appeal against the High Court's decision, which ruled the Prime Minister cannot use executive powers to trigger Article 50.